It is well known that wind speeds are not constant and may vary significantly from one moment to the next. It is important that control of the wind turbine (whether it is a variable speed wind turbine or a constant speed wind turbine) can be effectuated to adapt for changing wind conditions. Regardless of which type of control is chosen, adapting the control to a wind gust remains problematic. Especially a so-called “Mexican hat” wind gust, as defined in the standard IEC 64100-1 2nd edition 1999-02 can cause problems. Such a wind gust has been depicted in FIG. 1. A slight decrease in wind speed is followed by a sudden increase in wind speed. With the variable speed operation described before, the initial reaction will be to decrease the pitch angle (to increase the angle of attack of the blades). When the pitch angle has been decreased, a sudden increase of wind speed occurs. The pitch control will pitch at highest possible pitch rate to decrease the aerodynamic torque. Frequently however, the maximum pitch rate will not be high enough to properly react to the wind gust. This may lead to high loads on the wind turbine and may also lead to disconnection of the wind turbine from the grid. It is of course undesirable that wind turbines suffer high loads and frequently have to be disconnected from the grid due to wind gusts.
In the prior art, various attempts have been made to alleviate previously mentioned problems. US 2007/0067067 describes installing so-called SODAR devices capable of measuring the wind speed upstream of the wind turbine rotor. US 2009/0047116 describes installing a LIDAR device installed in the wind turbine hub, also with the goal of measuring the wind speed upstream of the wind turbine. The main disadvantage of these systems is that separate dedicated systems are needed, which represent an extra cost. US 2007/0124025 describes another system, in which the control of a wind turbine uses data from other wind turbines located upstream of the wind turbine. This system however depends on the presence of other wind turbines, and does not solve the problem for the wind turbines that are placed upstream from other wind turbines.
U.S. Pat. No. 6,909,198 describes a method for processing and/or predicting flow data of a flowing medium, in which from values of at least one flow parameter which are successively measured on a continuous basis at one or various locations in the medium, said flow parameter being characteristic for the speed of the medium, a time series is formed and updated which is subjected to a nonlinear deterministic prediction procedure on the basis of a locally constant phase space model for generating prediction values for the respective subsequent flow parameters, wherein a predetermined control signal is generated if the prediction values for an impending change in flow speed. Because this method is based upon a nonlinear deterministic prediction procedure, the computational capacity necessary in order to properly carry out this method is very high. The computer able to carry out the predictions may therefore be expensive.
Another prior art method is described in WO 2007/138138. According to this method, the pitch of the blade is controlled (as if under extreme wind gust conditions) whenever the instantaneous acceleration of the generator is above a predetermined threshold. A disadvantage of this method is however that the detection of a wind gust is reduced to a simple measurement of only the acceleration of the generator. Any other factor, that may indicate a wind gust is not occurring is ignored. Additionally, the criterion only takes into account a single extreme wind gust. Any other wind gust will not be detected.